Full Idea
In mainstream just war theory (Anscombe, Nagel, Walzer) the criterion of liability to attack is simply posing a threat. Since all combatants pose a threat to each other, they are morally liable to attack; because noncombatants do not, they are not liable.
Gist of Idea
Just war theory says all and only persons posing a threat are liable to attack
Source
Jeff McMahan (Killing in War [2009], 1.2)
Book Reference
McMahan,Jeff: 'Killing in War' [OUP 2009], p.11
A Reaction
McMahan says that the distinction between legitimate and illegitimate targets rests mostly on this basis. The problem is that a huge range of unarmed people can also pose various degrees of threat.
Related Idea
Idea 23598 You (e.g. a police officer) are not liable to attack just because you pose a threat [McMahan]